1. Field of the Invention
The present invention is related to a method of and an apparatus for establishing a reference point, that is, a highlight point and/or a shadow point, on gradation correction curve which is employed in gradation correction of image data.
2. Description of Background Art
A color scanner usually comprises a gradation correction unit for performing gradation correction on image data of an original image (or an image to be reproduced) having gradations in tone.
The gradation correction is performed according to a gradation correction curve. An operator sets up the gradation correction curve in the color scanner by selecting a highlight point and a shadow point on the original image, and by inputting into the color scanner density values of the highlight and shadow points for highlight and shadow density values) and halftone-dot area rates corresponding to respective highlight and shadow density values. Because the highlight and shadow points are selected only by the operator's judgement, the skill of the operator affects the quality of a reproduced image. In order to cope with this problem, several methods of automatically setting up the gradation correction curve have been proposed as follows.
Japanese Patent Laying Open Gazette No. 60-87594 discloses the following method: A brightness signal is obtained by adding up color-component signals R, G and B of a selected image point while weighting the color-component signals, respectively. A cumulative histogram of the values of the brightness signal is then prepared. On this cumulative histogram, the upper limit value (or highlight density) and the lower limit value (or shadow density) of the brightness signal are obtained corresponding to respective predetermined cumulative frequencies, such as 1% and 99%. Each of color-component signals R, G and B of an image is normalized with the upper and lower limit values in the gradation correction operation.
Japanese Patent Laying Open Gazette No. 60-87595 discloses a method in which the upper and lower limit values are obtained for each of the color-component signals R, G and B while the cumulative histogram is prepared for each of the color-component signals.
Japanese Patent Laying Open Gazette No. 62-11570 discloses a method in which highlight and shadow density values are obtained for each of the color-component signals R, G, and B while the cumulative histogram is prepared for each of the color-component signals on the basis of an image signal obtained by preliminary scanning of an image.
In the above methods, the highlight and shadow density values are related to predetermined cumulative frequencies, such as 1% and 99%, respectively. Accordingly, if the original image is discolored or has color fog, the color tone of a reproduced image becomes similar to that of the original so as to seem unnatural.